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19 votes
Accepted

Why is my hydrogen energy not equal to -0.5 hartrees?

Yes, the exact orbital energy of the hydrogen atom is $-1/2\ E_h$. The problems in the calculation are as follows. First, you are running ROKS. ROKS has many definitions in the literature; PySCF ...
Susi Lehtola's user avatar
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12 votes

Why is my hydrogen energy not equal to -0.5 hartrees?

The exact non-relativistic ground state electronic energy of a hydrogen-like atom, assuming that the nucleus is "clamped", point-sized, and with an infinitely large mass compared to the mass ...
Nike Dattani - No Free Time's user avatar
11 votes
Accepted

Does the size of atoms influence the speed of DFT calculations when comparing systems with almost the same number of atoms

For local basis set software, the size of the atoms should not affect anything, only the number of electrons; however, I note that your example software tags (VASP and QE) are plane-wave based. In ...
Phil Hasnip's user avatar
  • 8,012
9 votes

What files are important while taking backup of a VASP calculation?

I'll separate the importance of the files to 2 categories. Purely for data analysis purposes The CONTCAR files are definitely important for relaxed structural ...
CW Tan's user avatar
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8 votes

Understanding algorithm of Quantum ESPRESSO from scratch

Most plane-wave codes are really very similar, so learning one is learning them all. There are important differences between pseudopotentials, PAW, LAPW, etc, but you can worry about that later.I ...
Tyler Sterling's user avatar
8 votes
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Comparing results of various DFT calculations in Quantum ESPRESSO

There are two ways to approach this problem. You can either perform a vc-relax calculation which will optimize/relax the atomic positions as well as the cell ...
Abdul Muhaymin -Free Palestine's user avatar
7 votes

Quantifying relativistic effects in DFT calculations

In reality, the contribution to an energy due to relativity, would be the difference between the "exact" energy with a model that captures all relativistic effects, versus a model that ...
Nike Dattani - No Free Time's user avatar
7 votes

Have quantum computers been proven to speed up any matter modeling algorithm?

FCI (Full Configuration Interaction) The FCI expansion of a wavefunction allows us to obtain the exact energy of a system given a basis set with $N$ spatial orbitals. On a classical computer, for a ...
Nike Dattani - No Free Time's user avatar
7 votes

Number of k-points for unit and super cell

Why is plane wave basis set is good for 1s and 2s orbitals, but for higher convergence is poor? I guess by "higher" you mean higher principle quantum number $n$ (i.e., 3s, 4s-orbitals). For ...
Chengcheng Xiao's user avatar
6 votes
Accepted

How are 2-electron integrals usually computed?

Integral calculation is discussed extensively in the book Molecular Electronic-Structure Theory by Helgaker, Jørgensen, and Olsen. 2-electron integrals are usually computed analytically with the ...
Susi Lehtola's user avatar
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6 votes

Stopping an ongoing relax calculation with provision to continuing the calculation later on from that checkpoint

How can I interrupt an ongoing relaxation calculation in Quantum ESPRESSO on my system, while ensuring that I can resume it later from the same point? Short answer you can't. Apart from getting the ...
Atom's user avatar
  • 1,005
5 votes
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What is the correct way to set up occupation for single atom in CFOUR software?

Your setting is incorrect for the dominant Slater determinant when calculating the electronic ground state of an O atom: OCCUPATION=2-1-1-1-0-0-0-0/2-1-0-0-0-0-0-0 ...
Nike Dattani - No Free Time's user avatar
5 votes

How to maintain constant liquid density during AIMD with NPT ensembe?

In an AIMD simulation the system size will be very small, so a constant pressure ensemble will entail relatively large volume fluctuations. You should stick to a constant volume simulation unless you ...
Shern Ren Tee's user avatar
4 votes

Which lattice parameter should be used, the one obtained by vc-relax or the optimized value acquired through the Birch-Murnaghen equation?

I will personally recommend to use the result (lattice parameter) after the fit with the Birch-Murnaghan equation, that what people do especially the bulk modulus will be included as a result and ...
M06-2x's user avatar
  • 916
4 votes

How can I calculate the elastic properties of a unit cell and a supercell?

There is new tool called ElasTool. It is a Python-based toolkit for computing the second-order elastic constants (SOECs) and mechanical properties of crystal systems. It is designed to integrate with ...
Thomas's user avatar
  • 9,132
4 votes
Accepted

What is the gold standard ab initio pair-potential between two water molecules?

I'm not an expert, but I believe Paesani's many-body MBPol force fields Acc. Chem. Res. 49, 1844 (2016) afford some of the most accurate descriptions of the interactions between water molecules. The ...
Susi Lehtola's user avatar
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4 votes
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How to get DOS for each iteration?

Since FLEUR was mentioned in the question I give an answer for that DFT code... In FLEUR we distinguish between SCF calculations and calculations on the DOS for a density that is already available. In ...
Gregor Michalicek's user avatar
4 votes

How to get DOS for each iteration?

Usually, the density of states (DOS) is calculated for the converged SCF calculation. However, to address your unique goal you might try TURBOMOLE. Since you mentioned QE, ABINIT, etc. I assume you ...
XYZ CIF's user avatar
  • 41
4 votes

Has non-orthogonal CI (NOCI) been implemented in PySCF?

NOCI is not available in the main branch of PySCF That is why your question on the PySCF GitHub page which you asked on the same day as this question (next time please let us know if you are "...
Nike Dattani - No Free Time's user avatar
4 votes

How to remove f functions from a cc-pVQZ basis set in CFOUR format?

If you use the Basis Set Exchange anyway, I recommend you have a look at the Python library which makes it quite simple to modify basis sets in any way you want. The code below does what you want but ...
Susi Lehtola's user avatar
  • 19.7k
4 votes

vdW Corrections for Comparing the Effects on Small Molecule Adsorption

Using DFT, DFT-D2, DFT-D3, and DFT-4 will all give you slightly different energies at each geometry, and therefore you will get slightly different geometries if you do a geometry optimization with ...
Nike Dattani - No Free Time's user avatar
4 votes

How to determine energy convergence threshold and force convergence threshold in Quantum Espresso

The parameters etot_conv_thr and forc_conv_thr determine the convergence thresholds for the ionic minimization. When the change ...
manju9's user avatar
  • 248
4 votes

Different results from MP2 calculations on same geometry in ORCA

Have you confirmed that the geometries for each are exactly the same? If the second calculation took a single step, its possible it moved the geometry very slightly. ORCA's default convergence ...
Tyberius's user avatar
  • 15.5k
4 votes

Explain ab initio molecular dynamics like I'm five

As you suspected, molecular dynamics (MD) is simply propagating Newton's II equation of motion ${\bf F}_i = m_i {\bf a}_i$ for the nuclei with masses $m_i$ at positions ${\bf r}_i$, velocities ${\bf v}...
Susi Lehtola's user avatar
  • 19.7k
3 votes
Accepted

When orbitals are labeled based on their irreps in D2h, how are the orbitals ordered for an N atom?

"Nitrogen atom has a point group of Dinfh, but ⁠CFOUR uses a lower symmetry of D2h instead" The point group of the N atom (or any atom) is $K_h$, not $D_{\infty_h}$, the latter would be ...
Nike Dattani - No Free Time's user avatar
3 votes
Accepted

How to remove f functions from a cc-pVQZ basis set in CFOUR format?

The CFOUR basis format is laid out on their website, but it might be in a little more detail and a little more abstract than you need, so I'll walk through your example of Boron cc-pVQZ to make it ...
Tyberius's user avatar
  • 15.5k
3 votes
Accepted

Computing the kinetic energy of a molecular orbital

Looking up the above paper, The value of the kinetic energy U for each orbital in the initial state (usually the ground state) of the target is a theoretical quantity evaluated in any atomic or ...
Susi Lehtola's user avatar
  • 19.7k
3 votes
Accepted

Different results from MP2 calculations on same geometry in ORCA

Using ORCA 5.0.4, I ran calculations based on the input file provided. As is, the orca.relaxscanact.dat list: ...
TAR86's user avatar
  • 1,697
2 votes

Energy of formation Vs Formation Energy Vs Heat of Formation Vs Energy Above Hull differences

Since you mentioned the Materials Project, I assume you are talking about computational results. Formation energy is the energy with respect to some reference states. This is used for example with DFT ...
kpoint's user avatar
  • 139
2 votes

ASE: How to plot the irreducible Brillouin zone and this bandpath from a bandpath object

Since you have not provided the XYZ file that you are using to get the atoms I will demonstrate how to plot the bandpath using a simple water molecule. Here is a sample code to get the bandpath plot. <...
Vandan Revanur's user avatar

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